Process of making stretch wrap film

ABSTRACT

A method of making a plastics stretch film comprises the steps of taking a cast or blown film of LLDPE at a temperature of between 50° C. and 100° C., stretching it in two successive stretching steps, the first step having a stretch ratio higher than that of the second step, to cause both plastic and elastic deformation of the film. The film is then relaxed to substantially release all of the elastic deformation and winding the substantially relaxed film into rolls. Preferably, the first stretch ratio is in the range 1:1.70 to 1:1.80 and the second stretch ratio is in the range 1:1.85 to 1:1.95, and there is a reduction ratio of substantially 1:0.85 between the speed of the film during the second stretch rolling step and the speed of the film during wind-up.

Stretch wrap films are used extensively in packaging to package discreteunits together to form a unitary package and are also frequently used toattach a package to a palette, for example. Stretch wrap film may alsobe used as wrapping to protect a commodity from the environment duringhandling and transport.

Stretch wrap may be applied by an automatic or semi-automatic machinewhich includes stretch rollers to pre-stretch the film before it iswrapped around a package. Alternatively the stretch wrap may be appliedmanually by applying the film from a roll supported on a simplehand-held mandrel. The present application has particular application tothis latter category of stretch wrap material, and also to film for useon machines in which the amount of stretch prior to wrapping is low orzero.

EP-A-0531021 describes a process for producing a stretch wrap plasticfilm in which the film is “cold stretched” at ambient temperatures tothereby plastically and elastically stretch the film to cold orient it.The cold oriented film is then allowed to relax to recover substantiallyits elastic deformation before being formed into a roll. Thisarrangement provides a film wrap which is substantially as economical,and may be more economical, in film usage when applied by a handwrapping device than other stretch wrapping films are when applied byautomatic and semi-automatic machinery including power pre-stretchingdevices. The process described in this patent specification has beencommercially successful but, in practice, it has not been possible toachieve consistently as high a stretch ratio as 1:4 suggested in thepatent specification without experiencing difficulties due to filmbreakage.

According to this invention a method of making a plastics stretch filmcomprises the steps of taking a cast or blown film of LLDPE at atemperature of between 50° C. and 100° C., stretching it in twosuccessive stretching steps, the first step having a stretch ratiohigher than that of the second step, to cause both plastic and elasticdeformation of the film, relaxing the stretched film substantially torelease all of the elastic deformation and winding the substantiallyrelaxed film into rolls.

Preferably, stretching is performed at a temperature of between 75° C.and 90° C.

By having a film in the temperature range specified above and stretchingit in two successive stages it has been found that it is possible toproduce more consistently a pre-stretched film. The film also hasadvantages in having a clearer, less opaque appearance.

Preferably the stretching of the film that occurs in the two successivesteps has a stretch ratio in a range from 1:1.5 to 1:2.5 for each step.More preferably, the first stretch ratio is in the range 1:1.85 to1:1.95 and the second stretch ratio is in the range 1:1.70 to 1:1.80.Preferably during the relaxing step there is a reduction ratio ofsubstantially 1:0.85 between the speed of the film during the secondstretch rolling step and the speed of the film during wind-up.

Preferably the temperature of the stretch film is substantially 80° C.during the stretching steps. The plastics film may be at thistemperature by carrying out the stretching steps at an appropriateposition downstream of a casting or blowing/extrusion production line sothat the method in accordance with the present invention is carried outin line with the film production. Preferably however the method of thepresent invention is carried out out of line with the basic filmproduction process and, in this case, the film is preferably re-heated,for example by being passed over one or more heated rollers immediatelybefore the stretching steps.

The plastics material particularly preferred for the stretch film of thepresent invention is linear low density polyethylene (LLDPE). Thismaterial is a copolymer of ethylene with a C₄ to C₁₀ olefin; forexample, butene-1,3-methyl-butene-1,hexene-1,3-methyl-pentene-1,4-methylpentene-1,3-methyl-hexene-1,octene-1, decene-1 or a mixture thereof. The alpha-olefin is usuallypresent in an amount from 1 to 10 weight percent of the copolymer. Atypical manufacturing process for the formation thereof is disclosed inU.S. Pat. Nos. 4,076,698 and 4,205,021. The preferred LLDPE has adensity ranging from 0.900 to 0.940 g/cm³. This material preferably hasa melt index of from 1 to 6. A multilayer film is also suitable, such asa multilayer film having a three layer A-B-C structure wherein the Alayer comprises LVLDPE, the B layer comprises metallocene LLDPE and theC layer comprises LMDPE.

A particular example of a process in accordance with this invention willnow be described with reference to the accompanying drawing which is adiagrammatic illustration of the process.

A plastics stretch film of 17, 20 or 23 μm thick material consisting of100% LLDPE sold under the trade description 17, 20 or 23 UP 050 by MimaFilms s.c.a. of 148 Route d'Arlon, L-8010 Strassen, Luxembourg is takenfrom a feed roll 1 through a nip formed between a first heater roller 2and an idle roller 3. The film then passes around the top of heaterroller 2 around the bottom of a heater roller 4 and around the top of aheater roller 5. All three heater rolls 2, 4 and 5 are provided with are-circulating supply of oil heated to 80° C. Heater rollers 4 and 5 aredriven in opposite senses at a peripheral speed of 65 metres per minute.The heater roller 2 idles. As the film passes over the heater rollers 2,4, and 5 it is heated to a temperature of 80° C. The heater rollers areformed from a material having a high heat conductivity. Chrome rollershave been found to be particularly effective.

The heated film then passes beneath a first rubber covered stretchroller 6 which has a peripheral speed of 66 metres per minute. A secondrubber covered stretch roller 7 is located very close to stretch roller6, 1 mm or less, and is driven in the opposite sense to the firststretch roller 6 at a peripheral speed of 126 metres per minute. Thisstretches the film with a stretch ratio of 1:1.91, or 91% stretch. Athird stretch roller 8 which is again located physically close tostretch roller 7 at a spacing of 1 mm or less is driven in the samesense as the first stretch roller 6 at a peripheral speed of 217 metresper minute. This stretches the film with a further stretch ratio of1:1.72 or 72% stretch. An idler roller 9 creates a nip with the thirdstretch roller 8 to prevent slippage of the plastics film. As the filmpasses between the stretch rollers 6 and 7 and stretch rollers 7 and 8it is stretched in two steps with a total stretch ratio of 1:3.3 or astretch of 230%. The close proximity of the stretch rollers 6 and 7 and7 and 8 prevent substantial necking-down of the plastics film materialduring its stretching.

The film leaving the nip between the third stretch roller 8 and theidler roller 9 then passes over a third idler roller 10 before passingaround the surface of a polished roller 11 which has a peripheralsurface moving in the opposite direction to the direction of movement ofthe film, before being wound on a core to provide an output roll 12driven by being pressed against the surface of the roller 11. The roller11 has a peripheral speed of 185 metres per minute and thus there is adifference ratio of 1:0.85 between the peripheral speed of the thirdstretch roller 8 and that of the wind up roller 12 which is sufficientto allow substantial relaxation of the film before it is wound up. Thus,the plastics film from the feed roller 1 is subjected to a total stretchof 1:3.3 followed by a relaxation of 1:0.85 leading to a total stretchratio of 1:2.85. The typical thickness of the stretched output materialis 6.8, 8 or 9 μm.

1. A method of making a plastics stretch film comprising the steps of:a) taking a cast or blown film of LLDPE at a temperature of between 50°C. and 100° C.; b) causing both plastic and elastic deformation of thefilm by stretching it in two successive stretching steps, said firststep having a stretch ratio higher than that of said second step to forma stretched film; c) relaxing said stretched film substantially torelease all of the elastic deformation to form a substantially relaxedfilm; and d) winding said substantially relaxed film in~to a roll.
 2. Amethod of making a plastics stretch film as claimed in claim 1, whereinthe temperature of said film is between 75° C. and 90° C. during saidstretching steps.
 3. A method of making a plastics stretch film asclaimed in claim 2, wherein the temperature of said film issubstantially 80° C. during said stretching steps.
 3. A method of makinga plastics stretch film as claimed in claim 1, wherein the stretching ofthe film that occurs in said two successive steps has a stretch ratio ina range from 1:1.5 to 1:2.5 for each step.
 4. A method of making aplastics stretch film as claimed in claim 1, wherein the stretching ofthe film that occurs in said first step has a stretch ratio in the range1:1.85 to 1:1.95.
 5. A method of making a plastics stretch film asclaimed in claim 1, wherein the stretching of the film that occurs insaid second step has a stretch ratio in the range 1:1.70 to 1:1.80.
 6. Amethod of making a plastics stretch film as claimed in claim 1, whereinthe stretching of the film that occurs in said first step has a stretchratio in the range 1:1.85 to 1:1.95 and wherein the stretching of thefilm that occurs in said second step has a stretch ratio in the range1:1.70 to 1:1.80.
 7. A method of making a plastics stretch filmcomprising the steps of: a) taking a cast or blown film of LLDPE at atemperature of between 50° C. and 100° C.; b) causing both plastic andelastic deformation of the film by stretching it in two successivestretching steps, said first step having a stretch ratio higher thanthat of said second step, wherein during said second step said film istraveling at a first speed; and c) relaxing said stretched film bywinding said film into a roll at a speed 0.85 times said first speed. 8.A method of making a plastics stretch film as claimed in claim 7,wherein the temperature of said film is between 75° C. and 90° C. duringsaid stretching steps.
 9. A method of making a plastics stretch film asclaimed in claim 8, wherein the temperature of said film issubstantially 80° C. during said stretching steps.
 10. A method ofmaking a plastics stretch film as claimed in claim 7, wherein thestretching of the film that occurs in said two successive steps has astretch ratio in a range from 1:1.5 to 1:2.5 for each step.
 11. A methodof making a plastics stretch film as claimed in claim 7, wherein thestretching of the film that occurs in said first step has a stretchratio in the range 1:1.85 to 1:1.95.
 12. A method of making a plasticsstretch film as claimed in claim 7, wherein the stretching of the filmthat occurs in said second step has a stretch ratio in the range 1:1.70to 1:1.80.
 13. A method of making a plastics stretch film as claimed inclaim 7, wherein the stretching of the film that occurs in said firststep has a stretch ratio in the range 1:1.85 to 1:1.95 and wherein thestretching of the film that occurs in said second step has a stretchratio in the range 1:1.70 to 1:1.80.